someone who is having clothes made for them puts on the clothes before they are completed to make certain they will fit.
这句话是关于“fitting”这个词的解释,整句话合起来就是fitting这个词义中的一个：试穿.也就是详解“试穿”的定义.
is having clothes made for them：已经为他们做好的衣服
整句话就是：人们在完全肯定衣服是否合身前,把已经做好的衣服穿上身,（看看是否真的合身）,这就是“试穿”.
【注】这是剑桥高级字典上的词条：fitting：noun （clothes）：{C}an occasion when someone who is having clothes made for them puts on the clothes before they are completed to make certain they will fit.
简单的说,老师让你用英语解释“试穿”这个词的定义,你就会用你题目上的这句话了.